1. Field of the Invention
This invention pertains to the field of direct siliconization of cast iron in cupola furnaces.
2. Description of the Prior Art
It is known that there are significant difficulties in the direct siliconization of cast iron in a cupola furnace when using charges containing a high percentage of steel scrap. Most often, significant silicon oxidation must be accepted. In this regard, aside from the economical disadvantages, it is especially inconvenient that the silicon oxidation also varies to a large extent. Therefore, a direct reproducible siliconization is almost impossible.
In the 1930's, it was attempted to add silicon carbide to conventional cast iron melts in the cupola furnace to influence the melt and the properties of the cast iron (see U.S. Pat. No. 2,119,521; Giesserei, 1962, 368, 376) but the results varied. In the second half of the 1960's, success was obtained in making available molded pieces of silicon carbide which made it possible to directly influence the composition, the melting procedure and the properties of the cast iron (German patent No. 1,583,262).
However, in the use of these otherwise fully satisfactory molded pieces of silicon carbide, sometimes unexpected and unforeseeable deviations resulted. It was then recognized that such phenomena, which are inexplicable in themselves, can obviously only occur if the charges contain significant amounts of steel scrap. These phenomena occur particularly in cupola furnaces without a lining. In these special cases, the otherwise fully satisfactory molded pieces of silicon carbide mentioned above do not seem to provide problem-free operation.